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A cave is an airfilled underground void, large enough to be examined in some way by man and formed by nature.

This is not the only definition of a cave, but definitely the most commonly used one. It is not precisely the way the term cave is used in common language. Especially it excludes all man made "caves" which are better called subterranea. Also mines are not caves.

Caves are formed naturally by geologic processes. There are hundreds of ways how caves can be formed, which are typically a combination of chemical processes, tectonic forces and atmospheric influences. The most common process of cave formation is karstification, which is the solution of rocks by rain water. First there are caves formed together with the surrounding rock, like lava tubes and blister caves. They are called primary caves. Then there are secondary caves, which are formed inside the rock by a process which removes material. Those processes are solution and erosion. Erosion is a mechanical form of weathering which is caused by running water, a cave river which works on its bed deepening it or waves (on coasts). This process is not depending very much on the type of rock.

Solutional caves form anywhere with rock which is soluble, and are most prevalent in limestone, but can also form in other material, including chalk, dolomite, marble, loess, ice, granite, salt, lava, sandstone, and gypsum.

Cave formation in limestone occurs because limestone dissolves under the action of rainwater and groundwater charged with CO2 and naturally occurring organic acids. The dissolution process produces a distinctive landform known as karst and characterized by sinkholes, sinking streams, and underground drainage. Limestone solution is the single most important process forming caves and the origin of probably 80% of all caves on Earth. The main reason for the overwhelming number of karst caves is the fact that much of the surface is built of limestone, but also the slowness of the soltution process. If it was faster, the life span of limestone caves would be much shorter and their number much lower.

Limestone caves are often adorned with calcium carbonate formations produced through slow precipitation, including the most common and well-known stalactites and stalagmites. This secondary mineral depostits in caves are called speleothems. Although calcite and aragonite speleothems are most common, there are virtually hundreds of different speleothems known. The world's most spectacularly decorated cave is generally regarded to be Lechuguilla Cave (New Mexico, USA).

Lava tube cave at Craters of the Moon

Caves can also form through the abrasive action of rivers, the wind or sea. Sea caves are very common at all coasts of the world, but as they are restricted to the zone where waves work on the rocks of the coast they are rather small. There are also some special cases; glacier caves occur in and under glaciers, formed by melting. They are also influenced by the very slow flow of the ice which tends to close the caves again.

Lava tubes are formed through volcanic activity. They are the most common primary caves. Lava flows downhill and the surface cools down and becomes hard. The lava now flows inside its crust, until the eruption ends. The liquid lava inside the crust flows out and leaves a hollow tube. The most important lava tubes are found on Hawaii (Big Island). Kazumura Cave near Hilo is the longest and deepest lava tube of the world and also the eighth longest cave of the United States.

Caves can reach considerable dimensions. Of the cave systems that have been discovered so far, the most extensive is Mammoth Cave (Kentucky, USA; 560km of passage). The deepest cave changes rather often, as every year new cave branches are discovered all over the world. The deepest caves of the last years are Voronya Cave (Abkhazia, Georgia) and Lamprechtsofen (Austria). The current (2003) number one is Gouffre Mirolda (Haute Savoie, France) with a height difference of 1,733m between the highest and lowest entrances. The largest individual cave room found is the Sarawak Chamber (Borneo, Malaysia; area 700m by 300m, height 70m) - big enough to accommodate 40 Boeing 747s or to fly a light aircraft through.

Caving and spelunking are a popular activities in several countries. But there are numerous restrictions which should be considered. In the United States, since most land in the eastern U.S. is privately held, permission must be obtained from the landowner to look for caves. In the western U.S., a permit for publicly owned lands may be required. Other countries have laws which generally prohibit the visit of caves, other allow it only in the company of local cavers. Some caves are closed completely for certain reasons. But even in accessible caves many laws apply in order to protect them.

Despite the legal matters, caves are generally very fragile and any visit harms the cave. Cavers developed their own ethics and even the casual cave visitor should obey the most important ones: do not destroy anything, do not leave anything, do not take away anything.

See also: Quarries, Mines, list of caves, Cave painting

The outside world viewed from a cave

External links

In addition to the usual meaning of the term there are three locations in the United States with this name. They are in Stevens County, Kansas, Lincoln County, Missouri and Pendleton County, West Virginia.

Caves is also the name of a commune in the Aude département, in France